A photo slideshow is a sequence, not just a folder of files with matching dimensions. Choose an opening image that explains the subject, follow with useful detail, and remove repetitive photos that add little. Decide the order before resizing so you can judge whether each crop contributes something to the story rather than simply filling another slot.
This page starts with our 1080×1920 vertical preset. That is a preparation choice for a tall photo workflow, not a guarantee about every TikTok account's available post types or upload limits. Check the publishing options in the app you actually use before preparing a large set.
Look for mixed source shapes. A landscape group photo and a close vertical portrait will not survive the same crop in the same way. If one image needs its full width, revise the composition in an editor or keep its proportions instead of accepting a crop that removes an important person or part of the subject.