I often find myself in the position of designing a website or other form of documentation for a client that requires use of his company's logo, but he'll only have the logo in the form of JPG when a transparent GIF or PNG is preferable, and even with the latter two it's virtually impossible to make any kind of significant adjustments except for cropping and overall color changes.
When consulting a graphic designer for any kind of design it's always better to obtain a copy of the original graphics' file if you perceive you'll use it again in the future for any reason. In other words if the product was designed in CorelDRAW then the client should try to obtain a copy of the original CorelDRAW file, even if he has no idea on how to use the program himself. Yet I can't remember ever designing a website for anyone who had access to his original logo unless he designed it himself. I think there are a number general reasons this happens:
1 - The graphic designer intentionally withholds the original file. In other words the graphic designer may retain the original file so that whenever the client needs a modification he'll have to consult him, or it can be a scenario where the client hasn't paid in full and as such the designer desires to keep copyright over his work. If you're a corporate or large-business entity it's advisable to even have the designer, if he's not an actually employee of yours, sign a contract giving you full copyright over the logo or whatever original work he has produced in your name.
2 - The client never asks for the original file. I believe this is the main culprit, as in this is what usually happens as opposed to the designer intentionally withholding information. The client may be ignorant that he even needs the original file, thinking instead that if modifications are necessary he would just consult the original graphic designer. But in some cases he may decide to use a different designer or even make modifications himself in order to save money, and if the original designer proves to be stubborn when asking for this data it can cause a serious headache as in having to get the work redone altogether.
I've found that most people seem to be shy in asking for the original file from a graphic designer after the work has already been done. Therefore it is ideal to let the designer know, even before the work begins, that you expect a copy of the actual graphics' file once he's finished. Some designers may charge extra for this, but I think in general once they realize the client is wise enough to even request such they'll just comply with little to no hassle.