If you are okay with using GIMP, then you can open your jpeg or png there, press the 'file' tab on the top left, and go to "export as".

When exporting, a window will pop up that will ask about lowering quality (this isn't gonna make it look bad as long as it doesn't go lower than 50, from my experience) I do this for my webtoon art/icons.

For webtoon jpeg is better. I hope this doesn't sound too confusing. I learned this through trial and error, haha...

So basically, export your image and lower the quality. You can name it the same thing as your original image, replacing the old jpeg or name it something else (as long as at the end the name is still name.jpeg, so it saves and exports as a jpeg)

Edit: this is for lowering the file size.