how do I reduce pixels for a photo?

You can use Preview to scale jpg, png, or HEIC images (and other formats). Go to Preview>Tools>Adjust Size and set the required percentage of the original image (see below). For example, a 50% scaling will reduce the number of pixels by 75% (i.e. 25% of the original image size).

- Pie Lover

that depend on which program you attempt to do so with