Same problem here. The funny thing is that it was working fine for a few days, then started getting this message.

There was a problem rendering your image gallery. Please make sure that the folder you are using in the Simple Image Gallery Pro plugin tags exists and contains valid image files. The plugin could not locate the folder: media/k2/galleries/1

For the record:

• Name convention is correct ( image.jpg )
• Above folders exist
• Images exist in the folder
• Folders are writable
• Images were uploaded directly within the .zip file and not within a folder in a .zip

Taa

I recently noticed ... it can happen even if everything else is fine but one or more image file got corrupted.

Solution ...

go to media/k2/galleries/ folder... copy the folder with the number shown in the error message to your desktop or anywhere you wish. Example: media/k2/galleries/244

Now go inside that folder (say 244) .... delete all the images... copy a single image inside this empty folder from your backed up folder in desktop and check ... if it's OK copy another and so on ... until you find the one that was creating the problem.