PNG screenshots are often several times larger than they need to be, and some upload forms will only take JPG.
Inspect transparent regions: they become white because JPG has no alpha channel. Keep PNG if a transparent background matters.
Transparent areas become white in JPG. PNG preserves existing transparency but does not remove a JPG's background.
PNG ignores the quality slider. JPG and WebP encoding can change detail; this tool has no lossless-WebP switch.
A JPG cannot be transparent, so any see-through areas of the PNG are filled with white. Look at the result on the page or background it is going on. If you need the transparency, keep the PNG, or convert to WebP, which keeps it.
PNG and JPG suit different things. A photograph is far smaller as a JPG. A screenshot, a logo or a diagram with flat colours and sharp text can actually be smaller as a PNG, and it will look cleaner. The tool shows the size before and after, so you can see which is better for your picture.
No. Changing encoding cannot reverse earlier compression or create missing pixels.
What can be opened depends on your browser. JPG, PNG and WebP always work, and a file of another type may need to be saved as one of those in another program first.
Pictures with flat colours and sharp edges compress very well as PNG. JPG is designed for photographs, so for a logo or screenshot the PNG can be the smaller file.