Start with dimensions before quality
If an image will be displayed at 900 pixels wide, there is not always a good reason to upload a 6000-pixel version. Reducing dimensions can produce the biggest size saving, often without a visible difference when the image is viewed at its real display size.
Then tune compression
Once the dimensions are sensible, tune compression. There is usually a point where further compression becomes visibly worse. Start around the middle, then compare the image at the size a real visitor will see rather than judging only from a 300% zoom.
Watch text inside images
Logos, infographics, and screenshots are more sensitive to compression than ordinary photographs. If there is small text inside the image, inspect it after compression. A picture can look great from a distance while its letters become uncomfortable to read.
Filenames help organization
Before uploading, give files understandable names instead of IMG_4382.jpg. Clear names make your own library easier to manage. There is no need to stuff keywords into filenames; clarity is more useful.
The workflow I use
I usually set a sensible display width, choose a format that fits the content, use moderate compression, and then open the image at its real viewing size. If it looks good there, I stop. Chasing the smallest number in the file-size box is not worth damaging the experience.
Test the image in its real page
The best comparison is not inside the compression tool alone. Put the smaller file where it will actually appear and view it on a phone and a larger screen. You may find that a much smaller file looks nearly identical at real display size, or that small embedded text needs more quality than ordinary photos.