TL;DR: If an image is too big for the Telegram standards, the Telegram bridge automatically sends it as a document to avoid compression.

Are your images being sent as files?

The Beeper connection itself itself doesn't convert images, but it has some size and dimension limits (based on Telegram standards) for when it sends images as a document instead of as a photo. Sending as a photo means Telegram servers will convert it to a JPEG and compress it. The current limits are 10 megabytes or 5 megapixels. Anything larger than this, and the media will be converted into a file.