The standard image format when exporting HTML or EPUB from InDesign in PNG because the PNG file format supports transparency (and more colors than a GIF).
In some cases (e.g., photographic images), PNG can produce larger files, especially if you choose the High Definition (HD) option when exporting from the in5 dialog.
So, what if you know your images don’t have transparency and you want them to be HD…but you also want to try and keep the file sizes smaller?
In that case, you can use the Object Export Options (found in the Object menu of InDesign) to add custom settings to any given image. Great so far.
But now, what if you have a lot of images? It can take a long time to open the Object Export Options for each one, navigate to the EPUB and HTML tab, and select from the drop down menus.
You can create an Object Style (Window > Styles) that only applies Export Options. And you can even narrow that to just the image type and quality.
Note how every checkbox for attributes in the image below has a minus sign (-), except the EPUB and HTML options. This means the style will let the object itself control those “pass through” settings. So only the image export settings are applied.
Now you can simply select the images in your layout that you’d like to render as HD JPEGs (or any other type that you’d like to create an Object Style for), and it’s one click to apply the Object Style.
If you’re targeting an EPUB, you’re set to export. If you’re exporting HTML5 with in5, just make sure you choose Use Object Export Settings from the Advanced section of the in5 dialog.
Here’s an IDML file with the style already created that can you can use to import the style into your own documents.