JPG to PDF
Turn JPEG and PNG images into a PDF, one image per page, in an order you set by dragging. Nothing is uploaded and nothing is re-compressed.
The images are embedded, not re-compressed
A JPEG goes into the PDF as the JPEG it already is. The format stores compressed image data directly, so there is no decode-and-re-encode step and no second generation of compression artefacts.
That means the resulting file is roughly the size of the images added together, plus a small amount of structure. It also means a photo that was already soft stays exactly as soft as it was, and no worse.
Fit the page to the image, or the image to the page
Two different jobs. Photographs usually want a page the same shape as the picture, so nothing is letterboxed. Documents you intend to print usually want A4 or Letter, with the image centred inside it.
Both are offered because choosing wrong is annoying in opposite directions — a photo album full of white borders, or a scan that prints at the wrong size.
JPEG and PNG behave differently inside a PDF
A JPEG is stored using DCTDecode, which is the JPEG algorithm itself — the compressed data goes in untouched. A PNG cannot be stored that way, because PDF has no PNG filter, so its pixels are decompressed and re-stored with Flate.
What follows from that: a PNG usually gets larger on the way in, sometimes much larger, since Flate on photographic data is far weaker than JPEG. A screenshot or a line drawing stays about the same, because that is the content Flate handles well.
PNG transparency also has nowhere to go. A PDF page has no alpha channel in the way a PNG does, so anything transparent is composited against white as it goes in.
Answers
Which formats work?
JPEG and PNG. Both are embedded directly without re-compression.
Will the images lose quality?
No. JPEGs are stored as-is, so there is no second round of compression.
Can I set the order?
Yes, by dragging the thumbnails before you build the file.