I can't add any pictures to my MSN display
It says "The file you selected is not a valid image file. Please choose another file.
Any way to get rid of that message? it's really annoying!

Ensure in format of the file. It should be jpg, jpeg, png, which are official formats of display picture. It could also be occured by corrupted images. Make sure that isn't a corrupted one.

And how big display picture is it? If you have installed StuffPlug (or any other plugins/add-ons), try removing it and then setting that picture again.

It didn't let me to add any pictures at all..
Mostly my pictures are randomly sized like 1024 x 768 etc.
About 5 MB max.

However when I used "change my display picture..." option (under display pic, the arrow..)
Then it didn't let me to add that picure either but when I tried to add another one which it didn't let before then it accepted it and then I tried to add the one I tried first then it accepted it too.

Very weird.. happens occasionally.
However I don't know how to fix it permanently, always some messing with it

Try taking 96x96 crops of an image you'd like as a display picture, or resize an image to 96x96 (if its square) and saving them in the .png format, as this is what they are converted to when you import them anyway.

also, try using smaller file sizes. 5mb seems a bit much