I updated iphone 14 mini to ios 17.3. now I can't seem to post photos online

I wonder if this is a JPEG (JPG) vs. HEIF thing.

JPEG is a picture format that's been around a very long time. HEIF is a newer one – and so photos in HEIF format are compatible with a narrower range of applications. The advantage of HEIF is supposedly that it delivers better quality with the same amount of compression; or more compression for the same amount of quality.

"Though capturing in HEIF and HEVC format is recommended, you can set … devices [which support these formats] to capture media using the older formats, which are more broadly compatible with other devices and operating systems."

Using HEIF or HEVC media on Apple devices - Apple Support

I'm guessing that your iPhone 14 was set up, by default, to use HEIF. Although I don't know how your iPhone 7 was set up, it might be worth telling your iPhone 14 to use the "More Compatible" formats (as described in the article). After that, take some new photos and see if you can upload them.

iOS updates generally aim to enhance user experience, boost security, and fix bugs rather than disrupt core features. Posting Photos online issues after updates might stem from underlying, unmentioned factors. Updates won't typically alter personal data but may refine settings or features. Backup before major updates for safety. Review Apple's update details and backup advice for a smooth process.

Kindly consider checking your settings and addressing any device issues before attributing the issue to external factors.

See what each update does: About iOS 17 Updates, About iOS 16 Updates, About iOS 15 Updates, About iOS 14 Updates, About iOS 13 Updates, About iOS 12 Updates, and so on...