Considering that I added some posters without problems, it remains to ask:

1. What movie did you try to add a poster to?
   
2. In which language?
   
3. What language is defined in your profile?
   
4. Is your phone an Apple or Android?
   
5. And which browser?

1. I've gone on the pages of multiple films to see if it was just a few which it didn't work for, but the button to add a poster hasn't been anywhere on the screen for any.

2. I've tried various languages.

3. English.

4. Apple.

5. Safari, Google and Chrome.

What's not showing is the rectangular box at the top of the screen which contains the add poster button with the cloud symbol. It's not at the top where it was before or anywhere else on the page.

@LazilyLethargic Are you on mobile? The upload buttons were removed on mobile, although they might return. The thing is, none of the edit pages have been tested on mobile as they are not a supported platform for editing.

I rather not have upload functions on mobile honestly. Because its almost impossible to tell the full size quality.

I see, I didn't know they were removed though I hope they return. It's pretty inconvenient to have to go on to pc every time you want to upload an image and if you don't have access to a computer then you can't.

Honestly, the issue @alltimemarr mentioned is exactly why it was removed. I can't imagine a good way of ensuring quality control and how most users would be able to adhere to our image contribution guidelines from a phone. Our moderators have to spend a lot of time cleaning up images because even as it is, a lot of the image contributions simply aren't good enough.

Two issues right off the top of my head would be maintaining proper aspect ratios (ie. the need to crop images, which means an image editor on your phone) and judging the clarity/bluriness of an image on a high DPI screen like a phone (because 100% is not 100%). Those two issues are super hard and difficult to do "right" on mobile.