Help reducing map size (mb)

I have recently purchased some maps (jpg format, 23mb) and I am having problems reducing their size to fit the 5mb limit for Roll20. I have changed the format to gif which has taken the size down to 6.5mb but I wonder if anyone has any suggestions on how to further reduce the size.  Thanks. 

If the map has vast swaths of the same color, then you may want to try PNG as it is optimized for screen shots (and most screen shots have large areas of white, or black, or whatever). If its a more photorealistic map, then stick with JPEG, but make sure its not needlessly high resolution.  Roll20 likes their grids to be 70x70px, so I would measure the grid that is overlayed on the map (if any, if none, measure a square that should represent a single grid block) and scale the entire picture such that the grid size is 70x70. After the scaling is done, save it out as a separate JPEG (don't erase your high quality source file!) but play with the quality settings of the resulting JPEG.  I find that quality settings around "50" reduce the file size greatly, but don't impact the quality a huge amount.

I wouldn't go to gif, it's not well supported on Roll20 and it's limited color. Depending on the resolution, scaling is your best bet. Do you know what the image size is in pixels and in tiles? You can use that to calculate the size of a tile in the native image. If it's larger that 150x150 tiles, you can scale it down. Without visual loss. Example: if your image is 6000x6000 pixels, and 10x10 tiles, the tiles would be 600x600. Youvould scale that down to 25% size (150/600 = 1/4 = .25) without any noticeable loss on the VTT. 

I'll try out your suggestions, the maps are from <REDACTED>, quite photorealistic. 

These are 300 dpi, CMYK, jpeg image files. 300dpi is appropriate for printing.  Monitors are generally considered to display at ~72dpi.  You should be able to drop these down to 24% of their current resolution and still have a good looking board.

Another option is to cut them into sections.   I did that to one of the map in the starter set.  To make it easier, if the map is really huge, keep the pixel dimensions of the upper left sections in multiples of 70 to make lining them up easier.  The far right, bottom, and bottom-right sections can be any pixel dimensions.

The Aaron said: These are 300 dpi, CMYK, jpeg image files. 300dpi is appropriate for printing.  Monitors are generally considered to display at ~72dpi.  You should be able to drop these down to 24% of their current resolution and still have a good looking board. DPI isn't a meaningful measurement for images displayed on a screen. Image programs which store a DPI value in the image's metadata do so for use with printer drivers. (And/or their own default image scale/zoom.) What's important is that actual pixel dimensions of the image.

Ha!  While you're correct, in this case I think it's probably a reasonable assumption. 

For the stuff I make, I often "Map Stitch".  I produce mine for the Marketplace at 140px per Map Square, and then cut them usually in half or quarters.  Just make sure that your cuts line up with the Map Squares as well, lining them up across gridlines is a pain and the gridlines can cover up some of the little display quirks you can get with background colour lines showing up between the map tiles.