You can upload jpeg, png or gif files from your computer to your Constant Contact library if you want to insert them in your emails. With the basic library you have space for a total of five files, this includes images and documents! Try clicking the library tab of your account on the left and then click the upload to library button.
On the overlay window you will be able to drag and drop images to upload or browse your computer and select the files.
Just a note-- You mentioned emailing prospects. Due to Constant Contact's strict permission policy you are only able to email people who have subscribed to your mailing list previously.