In CORE, the company Email ID and logo in the header of invoices are brought forward from the Company screen. So, you can set and change both there (Settings > Company > Information). Check CORE Help Center for details.
The company logo displays only on invoice templates. Adding a company logo to reports can only be done via a custom report prepared by our Report Customization Team for a nominal fee.
Do note that the size of the logo image must be 1x1" and in the bitmap or jpeg format (.bmp/.jpeg/.jpg). You can use a free software for this, such as IrfanView. If you use a rectangle-shaped logo, the aspect ratio distorts the logo into a square. If you want to get it customized by BQE Software in case of non-square logos, you can email customreports@bqe.com for a quote and let them know which invoice templates you plan to use.
Also note that the sample invoices don't show your company logo. These samples can be found in Settings > Billing & Invoices > Invoice Templates and they show the BQE logo by default. When you create an actual invoice using the templates, your company logo is used, provided you have added that in your Company screen.