Delete image/blob content to reduce *.mdf file size? I have an old SQL Server database and find it unnecessary to keep old image files the users upload.

Currently the .mdf filesize is around 10 GB and database size is mainly blown up by the image files stored in each record row (I estimate 8GB are image files). If all blob/image files older than 1 year old are deleted. Is it possible to drastically reduce the .mdf filesize to 2-3 GB?

If this works, I'll schedule a script to delete all old blob/images at the end of each year, and just keep images for the past 12 months.
Yes, deleting the unused images will free up space inside your DB files.  To actually reduce the size of those files you will have to SHRINK them after the images have been deleted.  Here's a link to the MS Technet site for this process.  Shrinking the files will most likely introduce internal fragmentation into your tables and indexes.  A thorough REBUILD would be appropriate afterwards.  On a 10GB database, once a year, this shouldn't be an undue burden.