Please Help! How to shrink picture file sizes?

Apologies – have to type in English…

I’d like to share more traveling pics with everyone, but most of my pics are around 2-3Mbs big. It seems like it takes a long time to download on certain machines. Could anyone please share your tips on how to compress/shrink the pic file size before uploading, so it would improve the viewing performance/speed? I figured out how to change the dimensions around the pics, now I just need to learn how to upload them in smaller files. Your help is much appreciated!