Does this also happen when using Joomla Articles with images ?
Just a few pointers in here, the memory limit is not the size of the file, it is memory in usage while doing a process so If the resizing process is eating up too much memory, you usually get the error message, so in our case if you have lots of k2 item and cache is cleaned so when resizing all of those files you might hit the limit or if original files resolutions are high resolution then again to process such files memory usage gets high so you might hit the limit again...
Other factors are in case if something is wrong with the image type or certain images might be corrupted and causing errors, so it is easy to upload new images to test that out as well as checking image names using lowercase names no spaces etc.
We can findout if latest version is causing such problems and this can be tested by using earlier versions of the module if you have it available in your system. If that causes same problems then i am afraid problem is related to your server config or your setup.
This needs lot of testing as you can see, see you around...