Greetings!
I've been using your plugin for  over a year because it allows me to add mini galleries to articles. As of a couple of days ago, the "Thumbnails" stopped working. I've attempted to uninstall and reinstall the plugin, but this doesn't seem to work.
Here is my most resent article that I used the plugin on: www.rainbowbrite.co.uk/index.php?option=com_content&view=article&id=249%3Arainbow-brite-costume-by-ashley-s-mccormick&catid=25%3Acostumes&Itemid=64

This article, as well as all the previous older articles I used the code on have all stopped working.
I get a red X box, however when you click the box the image does show up, so it appears it's the thumbnails.

Have I made any recent changes to the site: No. I use the same layout/skin/theme, but change the images.
I did update my Joomgallery yesterday thinking that would correct the issue, but it did not, because the files are not associated with each other.

Previous topics for the past month  have had similar issues, but I have yet to see a resolution stated.
Any ideas? thank you!

Try to rename your images avoiding gaps, caps, slashes in their names...

There already aren't any gaps, caps, slashes in their names.
The file organization is usually the initials of the article and a number to keep them in order. No spaces to prevent confusion.

Also, this is affecting ALL the articles with the plugin, even the ones where it used to work.

www.rainbowbrite.co.uk/index.php?option=com_content&view=category&id=25&Itemid=64
www.rainbowbrite.co.uk/index.php?option=com_content&view=category&layout=blog&id=16&Itemid=61

What do you suggest now?

Then...
Check this:
forum.joomlaworks.net/simple-image-gallery-%28plugin%29/**solved-sig-does-not-show-thumbnails-but-image-lightbox-works-fine/
and this:
forum.joomlaworks.net/simple-image-gallery-%28plugin%29/thumbnails-not-showing-up!!-help-please/

Katia: You may want to check the links you provided.
Link 1: has no resolution
Link 2: is 3 years old, and it's resolution doesn't work.

I received an email from my host server saying the plugin "plugin_jw_sig" is causing too many 503 errors due to missing thumbnails, and to prevent suspension from the server I have had to remove the plugin completely.

I have no choice but to stop using the Simple Image Gallery plugin.

The first post gives solution related to permissions... The second suggest to check that you have GD library enabled...Did you check them?

What exactly is the error you get?