Looks like a rare document corruption or video card driver issue. The bitmaps are in memory but if you're not crashing Flash then it looks like you should re-import the source for those bitmaps to rule that out. Aside that, what video card/OS are you using and have you updated your driver to the latest version?
Thanks a lot for your reply!
I've already tried re-importing them, but the same thing happens..
My graphic card is: AMD Radeon HD 6490M 256 MB, I have OS X 10.8.2 and everything is up to date.
Not sure where to go from here..
So it's a laptop. Well nothing sticks out immediately although that's a curiously small amount of vid card RAM. How old is the laptop, what model is it?
Does it only ever do it to those two images or is it any image on the right side of the screen? Move your whole document over so other images are on the right side of the monitor and see if they also do it. If so it could be a video card issue.
Otherwise try to change the nature of those images. Right-click on them and in the properties tick off to 'allow smoothing'. If that doesn't help, change the graphic format (if it's JPEG, make it a PNG, or vice versa). Toggle the compression setting as well (PNG/lossless or JPEG compressed). See if something makes those images suddenly work to give you a lead on what the issue is.
Yes, it's a macbook Pro, 15". I forgot to mention that it has two graphic cards. Earlier the Radeon was in use, now the Intel HD Graphics 3000 384 MB is in use. I don't really know why it switched. I bought it in 2010. Is it really a small amount of vid card RAM? I've done projects way bigger than this one and never had an issue.
While multitasking (typing this reply and working on the project) I found out that they only do it if they're that small so I fixed the problem by making the pictures like 2 pixels bigger.. It didn't matter whether the files were JPEG or PNG. I still don't know why it did it, but at least it's fixed now.
Thank you very much for your replies!