I'm sorry to hear that you are having trouble resizing your images.
Images of less than 4095 x 4095 should be able to be resized OK.
What resize dimensions have you chosen (Max Width and Max Height)?
If you are not already using the default values (Max Width 1024 and Max Height 768), then please try the default values, at least for troubleshooting purposes, to see if this helps.
Otherwise, the images might somehow be corrupt. Even corrupt metadata (which might not be noticeable when opening the images in other imaging programs) could be enough to cause a problem.
Try opening one of your images in an imaging program (such as Photoshop) and resaving it (without editing it) to see if this makes a difference.
I hope this helps.
However, if you continue to experience difficulties, then perhaps you could upload one of your source images somewhere (e.g. Dropbox, Google Drive) and post a link to it here in the forum so that I can download the image and try to replicate the problem myself.
With access to one of the images that is giving you trouble, I might be able to figure out the cause of the problem.
Thank you.