Initially, I was going to suggest the following approach: Start out by creating a new 'blocking' layer and draw out vector shapes for the areas of the image that you'll be replacing or preserving (and perhaps a block for a new gradient). You can use a blend mode to better visualize the shapes against the original background. From there, you could preserve the letterhead, for instance, by copying the original background image to a new layer and adding a feathered mask, based on one of the blocks that you created:
At this point, though, you're still going to have to match the gradient, and that seems quite a task. You can attempt to do this by sampling corresponding points in your image... For this to work, though, you'd probably need to draw out a wider gradient rectangle than what I've shown (in the white border); I'd suggest going as wide as the approximate X coordinate of the first oval portrait. The angle of the gradient—which looks to be about 45 degrees—would then also need to be matched. And if the original gradient was based on more than 2 colors, that would complicate things further.
With this in mind, you might consider recreating the graphic instead. If you look closely, you'll see flaws in the original—in particular, some fairly unpleasant aliasing in the curved area beneath the portraits. (Click on the image below to view it full-size.)
So what you might do instead is copy the elements that you want to preserve—e.g., the portraits and the letterhead—to a new layer. Then draw a new gradient onto a vector object and place it beneath those elements. Meanwhile, the curved area in the bottom half of the header graphic looks somewhat like a very large, rounded rectangle—you might be able to use that shape tool to recreate the curve (without the rough edges).
Hope this give you some good ideas! (Otherwise, you might want to try the selection tools in Photoshop!)
Actually, I don't know why I didn't think of this approach first:
- With the Marquee tool, select an area of the gradient to be your "clone source".
- Copy (Command-C) and paste (Command-V) that selection to a new bitmap.
- Use the arrow keys to position the bitmap both horizontally and vertically, following the angle of the gradient. (Hold Shift to move in larger increments.)
- Copy the "clone" and reposition to cover more of the undesired object. And keep repeating.
This is just a start, but it should get you close. You'll likely need to clone at least one more area, and may want to try feathering or masking, as well. Also, to better see how the clone is blending with the background image, choose View > Hide Edges. (This turns off the blue borders around the currently selected object.)
Thank you for all your ideas, they've given me a good idea of how to tackle the problem. I agree that the image isn't that good in the first place, I'm really 'patching it up' with the intention of replacing it sometime.. The whole website needs a bit of attention.