Select the Save for Web (Legacy)... option under File then Export.
Depending on which optimized file format you select, there are several options available on how you want to best save your image for the web. We recommend you use either a JPEG or GIF format. Read more about Optimized File Formats for the Web here.
Reducing your image size will reduce the overall file size. The image below shows a width of 3000 pixels and a height of 2000 pixels. For web images, typically an image that has the longest edge at 1000 pixels will be large enough for your purposes. If the chain link is linked, both width and height will remain in the same aspect ratio when you change the other.
If you selected a JPEG Optimized File Format, then you can select a Compression Quality option.
If you selected a GIF Optimized File Format, then you can select a Color Reduction Algorithm, Dither Algorithm, and other options. We recommend that you stick with the default options, but you can read more about Optimizing GIF Files here.
Select the Export As... option under File then Export.
In the export menu, set your Format as JPG. You can modify the Quality of the image. A lower quality will have a smaller file size.
If you are uploading to an Omeka site, you will need to get your file size to 2mb or smaller.