Converting one picture is easy. Converting four hundred after a holiday needs a little planning, or your computer slows to a crawl and you lose track of which are done. These habits make large batches calm.
Work in groups
Everything happens in your browser, using your computer's memory. A very large batch of big photos can use a lot of it. If the page slows down, process 50 to 100 photos at a time. Finish a group, save it, press Clear and start the next. It feels slower to start, but it is faster than waiting on a struggling computer.
Use the zip
Each tool has a Save all as one zip button. Use it. One download is far easier to manage than a hundred separate files, and a zip keeps the photos together. When it finishes, unzip it into a folder with a clear name, such as the date and event, then check it before you delete the originals.
Settings apply to the whole page
On the Resize and Compress pages, changing a setting redoes every photo on the page at once. Use that to your advantage: drop in a handful of different photos first, adjust until they look right, then add the rest. Do not tune settings on the one easy photo and discover later that the dark ones suffer.
Check a few before you trust all
Open three or four results at full size: one bright, one dark, one with lots of detail. Look at edges, skin tones and sky gradients. If those look right, the batch is almost certainly fine. Check the file sizes too: a result that is bigger than the original usually means the setting is wrong for that job.
Keep originals until you are done
Do not delete the originals until the converted copies are saved, checked and backed up. Converting is not reversible in the sense that a lossy copy cannot get the lost detail back. If you want to free space on your phone, back up the originals first, then delete them from the phone.
Names, order and tidy-up
Converted files keep the name of the original with a new ending, so a folder sorted by name keeps its order. For an album that is going into a PDF or GIF, put the pictures in the right order before you press the button, since the output follows what you see on the screen. Afterwards, clean the downloads folder so you do not end up with three copies of everything.