Most capture devices (cameras and microscopes) capture images at low resolutions; usually 72 - 96 pixels per inch (ppi). These resolutions are far too low to print properly in print journals. While these images are captured at low-resolution, they are usually captured in enormous physical dimension. My digital camera captures images at 72 ppi but at 28 inches by 21 inches.
When this is the case, you can resize the physical dimension down, while increasing the resolution. This will enable you to provide images that retain their quality and meet print-resolution requirements.
What can you do when your images are too low in resolution?
Using Adobe Photoshop, open the image in question and select "Image" from the menu bar and "Image Size" from the drop-down menu.
Check the image resolution displayed in the Resolution box represented as pixels/inch.
Images generated at at 72 dpi at the final reproduction size
Unfortunately images generated at 72 dpi at the final reproduction size cannot effectively be increased using Photoshop. You can increase the dpi under the image, image size pull-down. However, this will not increase the quality of the image. It will add pixel information to the image that is essentially "noise". This may be considered "creating data" and an ethical violation.
Re-sampling images generated at 72 dpi that are greater than reproduction size-
Digital cameras and other digital devices capture images at 72 dpi at a much larger physical size. These images can be re-sampled to achieve the appropriate resolution. (See the example: It is 72 dpi and 28 inches wide).
Uncheck the Resample image box and then type the desired resolution "300 dpi."
This will effectively increase the images resolution while reducing the overall dimensions of the figure.
NOTE: If this method of Re-sampling gives an image that is too small in physical size, you can change the values without having unchecked "Resample image".
Change the resolution to the resolution to the required PPI
Change Document size to desired dimension.
This will give similar results as above, though this procedure has limitations. This procedure works best if the original overall dimensions are large enough (far beyond the needed dimension).