Photo editing is like fire starting: knowing the best way to do it is as important as being able to make do with what you have available at the time. Simple tools may yield simple results, but the job gets done when it might otherwise have been left unfinished.
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If the best camera is the one that's with you, sometimes the best photo editor is the one you have access to. Microsoft's Paint application doesn't have a circle selection tool, but you can make do. Open your picture in Paint. Choose the Oval from the Shapes menu, hold down "Shift" on your keyboard and draw the shape on the picture you want to crop. The Shift key will force the oval to be a perfect circle. In the Image section of the menu click "Select" and choose "Free-form Selection." Click and trace the edges of the circle with your mouse. When you're done click "Select" again and choose "Invert Selection." Select "Delete" to remove everything outside the circle you drew. You can clean up the rest of the crop job job with the eraser tool.
Online Photo Editor
Similar to using Microsoft Paint, online photo editors are not as powerful as dedicated apps, but they are widely available. Open a browser window with your online photo editor -- for example LunaPic (see link in Resources), import the picture you want to crop, choose the circle or oval selection tool and select the area you want to have as your cropped image. Choose "Crop" and then download the image to your computer.
Even though PowerPoint is a presentation app, it can still perform some basic photo editing. Insert the picture you want to crop into your PowerPoint presentation. Click to select the picture -- if it isn't already selected -- and choose the Picture Tools: Format tab from the menu bar. Click the button underneath Crop, point to "Crop to Shape" and select the Oval shape. If your original picture is square, the crop will be a perfect circle, otherwise it will be an oval with the same aspect ratio as the rectangle of the original image. Right click your cropped image, choose "Save as Picture" and save the cropped picture to your computer.
For serious photo editing, a tool like Photoshop is second to none. Click and hold the selection tool in Photoshop until other options appear, choose the Elliptical Marquee tool. Click and drag on the picture to select the oval shape; hold "Shift" to make the selection a perfect circle. Click the "Image" menu and choose "Crop."