Here's something I posted quite a while ago, but couldn't find it. Ahh, the power of Google!
To calculate the amount of memory an image uses, you simply multiply the pixel width and height, and then you multiply the result by the number of base colors (RGB - 3, CMYK - 4). Finally, you divide by 1048576 to get the memory usage in MB.
Here are some common image resolutions and their memory use in GD (assuming RGB):
800 x 600 - 1.37 MB
1024 x 768 - 2.25 MB
1200 x 1600 - 5.49 MB
Remember when using the above figures that the amount of memory being used by the rest of Coppermine must be taken into account, too.
If you are unable to change php.ini settings yourself, you can always ask your server administrator to change this for you. However, many administrators will be reluctant to do so, as this setting will affect everyone on a shared server. A higher memory limit requires reducing the number of people who can be hosted on the same server in order to maintain server stability. This reduces profitability, etc.
If you cannot change php.ini, you should read alternatives 2 and 3.
Alternative 2 (sensible):
Resize your images before uploading if you do not require high resolution images. This saves upload bandwidth and time for you.
Alternative 3 (workaround):
You may download one of many free programs that resize images. Then resize the images to a smaller resolution (like 800 x 600) and upload the resized images to Coppermine. Then use your FTP client to overwrite the images with the original higher resolution images.
The thing to remember about using any web software to resize your images: it eats up a lot of memory. The best thing to do is to resize your images first to a more web-friendly size, this way PHP won't consume as much memory. If you absolutely must have large dimension images, you can create the thumb_ and normal_ images with your image editing software, and then upload all of them via FTP. No resizing needed!
The best place to find ImageMagick do's and don'ts is at
www.imagemagick.org.