Metadatics offers the ability to convert and resize album artwork. You can access this function by Control-clicking on the album artwork pane, or from the Album Artwork panel.
Image Type: Choose between converting to JPEG or PNG.
Quality (JPEG Only): Selects the JPEG compression level. Higher quality results in better looking images but larger file sizes.
Image Dimensions: Allows you to enter new values for the width and height of the image. To resize the image by a percentage, choose "percent" form the pop-up menu.
Scale Proportionally: Checking this will ensure that the image keeps its original proportions. If you change either the width or height, the other value changes to maintain the proportions.