Here is a sample of JPG artifacts from a real image. This is a screen capture of Photoshop showing a portion of the same 1030x746 image (2.3 MB uncompressed), side-by-side TIF and JPG, both zoomed to to 300% size. The TIF file is on the left, and the JPG file with Quality 50 setting is on the right side (86,790 bytes, compressed to 4% original size - excessive! ). I normally always use 75-80 Quality (20-25 compression), because 50 is pretty poor quality.
There is a substantial loss of detail apparent, plus a generally high level of JPG artifacts. The 8x8 pixel JPG color blocks don't show much here, just detectable in the sky, but more so in the red and green cabin roof.
If the JPG were written at about 80 Quality, and if the images were shown at actual size instead of enlarged, the JPG would still look pretty decent, and the file size still quite small, but it still would not be acceptable as a master copy of the scan. The problem grows every time you compress and write the file again.
TIF FILE JPG FILE
In addtition to the artifacts around all the lines (JPG is not kind to sharp lines in photos), notice too that the red light on the top of the mast is no longer red. Wasn't enough of it to bother with I suppose.
For scale, below is the entire image reduced to 33% size. The large image above is the third ship back, the second red one. It was a Kodachrome slide.