We are working with a collection of archaeological and architectural photographs, so the size and quality of the files are important to us and those who will be viewing our site. Our preferred file type would be JPEG2000, but this file type is not compatible with Omeka. We currently have our photos in JPEG files (approx. 2MB each) which we compressed from TIFF files (approx. 50MB each), but even 2MB is a large size and may be slow to download for viewers and will require much server space for our large collection. Can anyone suggest an ideal file type (comparable to JPEG2000) which would optimize the quality and minimize the file size?
Has anyone fund a solution for this? I also have this problem.
For the time being, JPEG is probably your best bet for photographic images.
Depending on your needs, you might do some initial resizing of your images before adding them to your Omeka archive. Omeka will automatically create a set of smaller derivative images that will be used in pageviews (thumbnails, square thumbnails, and "fullsize" images). Users will only incur the extra download time if they click on a link to the "archive" or original image.
Support for JPEG2000 and other formats is on our radar, but there are many servers that don't have the support we would need (in ImageMagick, in particular) to read those formats.
As a possible workaround, JPEG 2000 has long been well supported in QuickTime (app and plugin). So one solution would be to export your high-res images to JPEG 2000 (jp2) and list them on the site as video/quicktime files with .mov or .qt extensions to prompt them open with the QuickTime app or plugin as needed. It's been a while since I've used this technique. It's described very well in the book "QuickTime for the Web".
This assumes you want to use JP2 only as a secondary, downloadable high-quality file format (i.e. you don't need on-site thumbnails). If you want to feed JP2 files to Omeka so that it can create various JPG image derivatives for your theme you're still stuck. A workaround here may be to initially upload your files in a small but compatible image format (say JPG) followed by a high-resolution ("JP2 as QuickTime") files. This will allow Omeka to create the thumbs and still bring you in well under your present file size budget. A note on your site can then say click here for high-res versions etc.
Two more ideas, one perhaps somewhat unorthodox. H.264 is a very efficient codec, even on still images. Experiment with compressing your high-res stills to .mp4 (using the H.264 codec). And you didn't mention this in your initial query but if you're also working with archaeological drawings (since these have lots of white space and thin black/gray lines), PNG is going to give you better compression and image quality than JPG (and likely even JP2).