With Fit image selected, a 1200 × 800 pixel image becomes a 900 × 600 point PDF page because every pixel maps to 0.75 point. After a 90-degree rotation, the same image becomes a 600 × 900 point page. Orientation and margin selectors do not add paper around Fit pages.
Compare the page with the source
Use one 1200 × 800 image and inspect the PDF page dimensions, not its apparent size at the viewer's zoom. The unrotated page is 900 × 600 points; rotate the queued image by 90 degrees and generate a new download to obtain 600 × 900. Each download still contains one page. The 0.75 mapping is a fixed pixel-to-point rule, not a reading of the image's stored DPI or a print-size guarantee.
Inspect the exact download
Open the downloaded PDF, count its pages, scan the first and last page, and confirm rotation, margins, sharpness and order. Keep the source images because a successful browser export does not certify suitability for a particular destination.