I encountered the same problem Unable to upload images frontend or backend through K2 Image tab. This client has its own server which has two joomla 1.5 sites, with K2 v2.5.5 and other components.

Whenever i tried to upload an image i got immediate blank white page. I set the maximum level of error reporting but again the pages were blank with no visible Errors. Tried a lot of stuff, setting php.ini variables, modifying permissions, etc. It didn't mather which picture format (png, jpeg) or file size (100KB or. 2,5MB) i tried, the result was the same.

After some time I found that in backend K2 settings->Images tab -> Advanced Image Settings -> Enforce memory limit (in MBs) for image processing was set to 32MB. Deleting this value and taking the server set limit fixed this problem for me.

Now i can only upload images with small size but at least I get php errors about time-out for larger images. Have to find a solution for that now.

I have the same problem, but the solution provided above doesn't solve it :( I already increase PHP upload limit and timeout, but still the same problem: white page when I upload images bigger than about 2 MB. Any idea why? Thank you so much in advance!!

It still causes memory related errors. Normally your images should not be that big. You can try increasing the memory size or trimming down your images.

Tyvm for your answer. Do you mean "memory_limit" in PHP settings? It's 500M actually...

Yaaaaah, solved it! This is my php.ini file:

Now also images bigger than 2 MB are uploaded without any problems :)

That's actually really, I mean really high so that is not the issue in your case.
I suggest you contact your hosting provider about this.

PHP's libraries for image processing can be quite intensive sometimes so that might cause an issue with your current hosting setup.