The most common reason you cannot upload an image is that it isn't an image.
Please do not try to upload a PDF or a Microsoft Word document.
Please make sure the image you are uploading is either a PNG, GIF or JPG, less than 5MB and 72dpi. Most problems that occur with image uploads are due to the size of the image.
The ideal Eventfinda image specs are 1170 pixels wide by 504 pixels high, with a 5-megabyte size limit and your event's featured image will display best if it is this size. This is the size of the landscape images you can see all through the site. The smaller images are simply those same images resized proportionally for display in lists or on the homepage.
You can include numerous images with your event listing however, only one can be selected as your featured image.