Generally, we need a resolution of 300dpi (dots per inch) or higher
Below are the most common types of digital images.
JPEG - Probably the most common image type found online, widely used to compress photos. JPEG refers to the type of compression that is made to an image. This compression reduced the file size so that it can display on websites faster than a non-compressed file.
PNG - Similar to JPEG, these are compressed image files but the difference is that PNG files are "lossless" meaning the image is better suited for graphics images, rather than photographs.
Vector graphics - The industry standard for professional logo designs, these images are deal for printing because they are composed of lines and curves and can therefore be scaled up to any size without compromising image quality. They are normally represented with the file extensions .pdf .eps .ai or .psd
Right click on your file and select Open with>Paint
Click on File>Properties and you will see the resolution displayed in the dialog box
Open your file in Preview.
Click on Tools >Adjust Size and you will see the resolution of your image.
You can also adjust the resolution here if required