Double check
Control Panel » Forum Permissions>Gallery Forum
Maximum size of gallery images
Just below the heading when you expand it, it will say something like
"If this forum is a gallery forum, this setting dictates how large each image may be. Max size set in php.ini is 2M, so you may not exceed this size."
The max size set in php.ini may be the culprit. What does yours say?
It sounds like it may be set to zero.
And BTW 100kb file is not very large
Also you will need to set a upload directory.
Control Panel » Feature Settings>>Attachments
Attachment Storage Directory:
(Click to test directory.)Don't forget to test!!!
Full URL to Attachment Storage Directory:
