You should resize your main product images before uploading if they are larger than 150kb and are large resolution, i.e greater than 800 x 800. You can edit these properties via any graphics editor.
Remember that product images uploaded in the gallery are not downloadable, so keep a copy handy in case you ever need it.
For images on general pages or your header, footer, columns etc. They should be resized and uploaded, never resize them after upload.
