First connect your account and then click 'Edit Profile'.
Scroll down to the 'Banner Image' in the public profile section. Or the card or logo if that's what you want to change. It is VERY important to add a new card image, as this is what users see on the marketplace.
NOTE: any image you choose to add should be LESS than 1Mb in size before you start to add it to the page.
Click the add image button in the middle of the current banner image.
Choose the image you want to upload - which should be sized in the ratio of about 1440 x 350 pixels.
Crop and edit the image to fit the rectangular aspect of the page
When happy, click the 'save' icon to save your image
To edit the logo, click the upload icon in the middle of the current logo
choose the image you want to upload - best size is 288 x 288 pixels
Crop and edit the logo - it will ALWAYS be square on the marketplace page, so choose a crop you are happy with.
When happy, click the 'save' icon.
To edit the marketplace card: basically very similar to the above instructions!
The dimension ratio should be approx 850x400 pixels
Don't forget to scroll to the bottom of the page to submit the changes. Otherwise they will not be added to your page.