Alex,
Your solution of renaming the file is correct.
The error message you received:
"Sorry! There is already an attachment with the same filename as the one you tried to upload. Please rename the file and try again."explains the problem. As more members attach more images to their posts, images with generic names like '001.jpg' will be rejected, because a photo of the same name has already been posted by someone else.
Please see this topic for more information -
Error messages when attaching files to postsRegarding the size of attached images, the current limits are fairly large - 4000kB per image and up to 20 images per post. So I doubt this was contributing to the problem.
However, please note that all attachments are automatically re-sized by the forum software to display at a maximum width of 900 pixels. This re-sizing process takes a little time and slows the page load time. The image also loses a little quality during this automatic re-sizing process.
If you have the ability to re-size images using some photo processing software
before you attach them, then it is best to post attachments using a width of 900 pixels. Your images will load faster for readers and appear better quality.
Dave