In OS X, you can use URL Access Scripting to download the file, as below (in this example it downloads it to your user Documents folder)
set docFolder to path to documents folder as string
set theFile to docFolder & ":temp file.jpg"
tell application "URL Access Scripting"
activate
set imageURL to http://mySite/myPic.jpg
try
download imageURL to file theFile with progress replacing yes
end try
try
quit
end try
end tell
You can then put a reference to the image file in your container field, as below:
tell application "FileMaker Pro"
set data of cell "Container" of current record of database 1 to file theFile
end tell
A possible problem with the above approach is that it does not put the actual data for the image in the FMP container, just a reference to it; move the original file, and you lose the image. AFAIK, there is no way to store the data in a FMP container using AppleScript without some help. If you want to store the data, you can use the GraphicsImporter OSAX (available for free at http://osaxen.com/graphicsimporter.html or at http://www.azug.gr.jp/~h-abe/freeware/gio/index.en.html), like so (after downloading the file):
set theImage to giconvert file theFile
tell application "FileMaker Pro"
set cell "Container" of current record of database 1 to theImage
end tell
Good luck!