The free tool Kev has linked to above is not always working (have we worn it out?
), so I've been to search for an alternative for you, and found this one, which seems to be even easier to use!
The easiest way is to use the online resizer here: http://www.shrinkpictures.com/
Select the image you want to resize and upload it, and choose the settings needed...
Select New Maximum Image dimension
I suggest 760 Pixels for image up to 3Mb original size, and 600 Pixels for images larger than that initially.
Set resized image quality (jpg compression) to Good
Press Resize button and wait for the result (it can take several minutes depending on the size of the image and your internet speed.
Once it's resized you'll see a new page, where you can download the resized image, and you have the option to delete the original immediately from their server too. If you don't delete immediately it is auto-deleted after 2 hours.
Your resized image should be less than the 125kb the board needs if you follow the settings above. Then you can just attach pics to your posts onto GMB.
Please contact me if you get stuck and need help with resizing images.