NextGen Gallery Thumbs

Installed NextGen Gallery and thumbs appear on the page where the gallery was placed, however, encountering a few problems.

First, cannot get all thumbs to appear; some are only empty boxes. A few of the photos that are 500 x 333 have visible thumbs, but those that are 700 X or 800 X pixels only appear as blank white boxes. Have followed all instruction on resizing but when I try to Create New Thumbs (100 x 75) in Manage Gallery all images show ‘A failure occurred’ Error 500 – Internal Server Error. Try again later. Not sure what’s going on. ???

Second, cannot get slide show to work. Downloaded and installed imagerotator as instructed but nada. ???

Third, cannot resize any of the images in my gallery. Downloaded and activated the Nextgen Resize plug-in but can’t get it to work. ???

Would be most grateful for any help. Really like the Nextgen plug-in but need to figure out what I’m doing wrong. Don’t know html and have very limited web skills.

Thanks.
John