Filemaker can resample graphic images, you'd need an image editor such as PhotoShop to do that. Don't know how much it would affect performance, but if you stored your images by reference in the container fields, your entire database would be much smaller.
It would be good to know what platform you are on, Mac or PC. On a Mac, this can be done with AppleScript. On a PC, the InsideScan plug-in can resize images, maybe others (Troi File can I think, but it costs more than InsideScan).
In either case you'd need to either do them one a time, via a script with a Loop, or export them all out, then do 'em all at once, with any image resize application (there are many that can do an entire folder, some free).
In either case it can be automated. But which depends on a few things. For one thing, an 'en masse' export into a single folder would require that the file names be unique. Which they may already be; use GetAsText (container) to set the current image's name into a text field to see. Or you may be able to add something to the name to make them unique. The advantage of this is that you would not need a plug-in or AppleScript, just a graphics app.
In any case, you'd want to export out the graphics, to save as full-resolution files. So really, you need unique names or a folder structure.
The FileMaker step to export each as a file is Export Field Contents. You would need to construct a full FileMaker syntax path for the proposed file, set it into a script Variable, then use that as the path.
Let us know what the image names are, or ask for me help with that.
Thank you both for your help !
i tried the plugin " InsideScan " and it does just what i need .
With a simple script, I will be able to resize all the pictures to a more adjusted resolution.
Again many thanks !