this is what I don't get

The answer to your question is simply that the screenplay is not great. The story and concept is not great.

Technically, they pulled it off quite well.. it looks good. But they needed better writing.

You are correct that there's a gaping timeline conflict. But they're counting on not many people noticing. It's part of the reason this movie will never be rated that highly.

The future never changes. Callie thinks the future changes and that the notes she sends to her past self can change the timeline, but they never do. She always sent a note telling herself to do the things she sent in her last note, except at the end when the last morning note she got was empty. She thought she could change the future, nothing can change the future.

Since nothing can change the future, there were never alternate timelines. The morning picture always showed what was going to happen next morning and the nighttime picture always showed what was going to happen next evening.

there is something else that I don't get:

On Tuesday evening my future self sends my past self Tuesday's winning scores. My past self receives these scores Monday evening. This is the first time my past self has received scores from my present self. Tuesday, my past self plays these numbers, wins, and has a fantastic day. But, my future self couldn't have won on Tuesday because they wouldn't have had Tuesday's scores. So their days had to have been different. How is this possible?

the affair thing happened months ago, before they discovered the pics, the camera was constantly taking pictures, remember?
she just hid the ones showing the affair and then used them later to manipulate the guys.

for the rest of it tho, i agree, there were issues..

My take on it is the following:
The plotholes do not result from sloppy writing. They result from the fact that time travel is impossible, and any movie or book that tries to tackle the subject runs headlong into paradoxes, and subsequently, plot holes.
Even the best of them can't avoid the paradoxes. No matter how you tackle the subject, you can't overcome that basic fact.

Some try to side step the paradoxes - as Time Lapse does, and some of them run with it, and purposely confuse the issue with even greater paradoxes - like Predestination. The paradoxes don't stop me from enjoying the movies - I do like all kinds of Sci-Fi and time traveling tales - but that is what they are - fictional tales.

There was one scene where Jasper took a picture of the picture with his phone camera. I was really interested in that picture, but they never made anything of it. There was an opportunity for some mind-bending developments. Like the phone camera could not take a picture of the future picture, and instead took a picture of the picture as though it was the present moment. Future pic is 24 hours from now but phone camera pic of the future pic = present moment. Not that this would have resolved the paradoxes.

Speaking of which, what if they put the future pic in front of the camera, and took a picture of the picture? Paradoxes galore - one of which is, they could not do so unless they had already DONE so and seen the future pic in the pic. But if they had done so, they should then see tomorrow's future pic in today's future pic, thus revealing the scene 48 hours hence. But once they tried that they'd be locked into that strategy, and always have to take a pic of a pic of a pic of a pic, etc, and wind up learning nothing about the future at all.

The real paradox seems to be, once the first picture is seen, all free will is demolished because then you have to be true to that pic and everything you do is to make that scene happen - you're always living to make the scene in tomorrow's pic a reality. The ONLY time you have ANY free will is the short space of time between when the picture is taken and you walk to Mr. B's apartment to look at it. If you never look at it, you can begin living your own life again.

She had sex with Jasper before they knew about the machine remember? She told Finn it was from before

I think someone already mentioned this, but just to reiterate: the affair happened. It happened way before any of the stuff we see in the movie. It happened during their drunken charades night that they mention several times; in fact, they talk about it when they see the very first picture with the lamp knocked over and they're trying to figure out when it happened. Callie just takes the photos of the affair off the wall and hides them because her future self told her too.

And no, it's not her present self sending messages to her past self; it's her past self getting messages from her future self and then, like they were doing with all the other photos, recreating the scene for the 8 a.m. shot. It's the exact same thing they were doing with the dog race results. And she doesn't change anything. All the photos happen exactly the way they came out of the camera. That's the whole point.

We find out suddenly that not only does Callie understand far more about the camera than the others, but she has been manipulating and correcting their events the whole way.
Instead of being treated like a big reveal, it comes off more like a sloppy script error. So yes, badly done.
And yet, it left the possibility that Callie could have fixed everything just the way she wanted it, if only the cop hadn't stopped her.