One can copy each item of a page to a new page with the same name. This way history is left behind. This can be useful when unnecessary image history makes a page slow to load.
Wiki stores edits to image items in the journal. This is both correct and inconvenient. The image and history travel with the page as it moves through the federation. That's the good part. The bad part is that many edits makes the journal fat with copies of the unchanged image.
The workflow described below is a workaround until image asset management is added to wiki that is both correct with respect to federation and efficient in storage and transmission resources.
# Squeezing Images
Drag the image to the desktop. Open it with an image editor. Adjust the size to a width of 400 to 500 pixels. Save as compressed jpeg.
I edit images with Preview. Adjust Size is on the Tools menu. Save As will offer to change format and compression level. In newer versions one must hold the option key to see this version of Save. I use two ticks short of maximum compression and get uniformly good results.
Drag the image back to an unwanted page in wiki. Edit the caption there. When satisfied, drag the image with caption to its proper place in wiki.
# Squeezing History
Create a new page with the same name on a new site. Drag the old page to the new site so that they are side by side in the lineup. Drag each paragraph from old to new. Optionally add a note explaining why discarding history was appropriate.