The wrong format either degrades the image or inflates the file. What to use when.
Images with smooth colour gradients, like photographs, compress well with lossy methods. Logos, text and screenshots have hard edges, and lossy compression makes those edges messy. What the image contains is the first thing that decides the format.
JPEG cannot represent transparency, so saving with a transparent background fills it with white or black instead. To place a logo over another background you need PNG or WebP. Backgrounds turning unexpectedly black in a conversion tool almost always trace back to this.
Lossy formats lose a little more quality every time you open, edit and save again. Keeping one copy of the original and editing from it each time avoids the accumulation. Lossless formats stay identical however many times you re-save.
If the other person needs to edit it, send an editable format; if they only need to read it and the layout must hold, send a PDF. Document files can carry author information and revision history, so it is worth checking before sending anything outside.
Reducing dimensions before changing format has the larger effect. People often use full-resolution images where a screen-sized one would do, and resizing alone cuts the file sharply. Adjust quality next, and change format last.
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