This is the part of the process which I do on a day-to-day basis. I use software called InDesign - part of the Adobe Creative Suite software - to create the Email graphics. As explained we previously created templates during our creative overhaul which speeds up the design process & ensures my designs are consistant with my colleague's work.
The screenshot on the right is from Indesign, the top half shows the templates we use. The magenta circles show where we would drop imagery in. Circles indicate price roundel size and position, there is an asset 'toolkit; that contains all the price roundels, branded font and other elements we can quickly grab from to place in our work.
The bottom half shows the 'pages' of our design. Indesign is traditionally a print based design software for magazines & desktop publishing. However we find it's also useful for digital work. I must consider both desktop and mobile graphics. Emails feature a different sized hero image between the 2 platforms for best user experience.
Most other graphics can work on both platforms.
Upon creating a new page in our document, I select a master template from the above section, then I would drop the required image or product in that frame along with the price roundel if required. Product images require cutting out in Photoshop using the pen or magic erasor tool and saving as a PNG so it has a transparent background, before using in my design.
Using Indesign allows me to create all elements in one document by using pages within a previously created template, and exporting them as JPEGS at once. Photoshop or Illustrator would require separate working documents for each element and would be time consuming.