You may have requirement for designing an ecover. You can think for hiring an ecover designer as ecover designer has following advantages:

An ecover designer will be able to bring the unique look for your ecover. He can use various design and image editing software to give you output. An expert designer naturally has collection of stock photos, custom drawing brushes, custom fonts etc. that are required for professional design. You can get the final product in your desired size and shape.
An ecover designers portfolio helps to get idea about his past work. You can get an assumption about the quality of output he will produce for you. You can suggest some of his past design styles in your own ecover design if you like the creative ideas.
Expert ecover designers quickly understand your needs and can design a stunning ecover in less time.
Your ecovers should match with page color and web page elements. Color and look of ecover should be designed in that way. Custom ecover design can change your ecovers look and feel to match with your sites look.
You may need to change the look of your ecover in future. Ecover designers keep the raw image file for rework. Further modification of your designed ecover can be done easily.

However, custom ecover design has certain drawbacks:
A custom design of ecover can increase your ad budget. A high return on investment from your product sale through ecovers may be enough reason to invest for ecover design
A professional ecover designer has too much workload. Getting your ecover designed within fixed turnaround is really tough. Getting a quality output at last may be a good compensation for time extension.
Quality of your ecover design may vary depending on expertise of designer. You have to be continuously in touch with your designer. You have to check if the designers creative idea is meeting your requirements.

An ecover designer can charge you a high price for his design, but you get a high quality graphic in the end. And if your designed ecover sells your ebook or CD/DVD more, then there is no reason to step backward. Hire an ecover designer and tell your needs properly. Get your ecover within a short turnaround to kick start business with a boom!