For scientific publications, you need to submit high quality images: high resolution, and preferably in vector format. I like to make most of my figures in R (Rstudio) because of the amazing options packages such as ggplot2, grid and cowplot offer. Unfortunately, standard outputs are not of great quality: 72 ppi, whereas most journals seems to prefer 300 (Plos ONE, Scientific Reports) and even 600 ppi (ESA). Fortunately, when using ggplot2, there are straightforward ways to achieve this, as well as getting output formats such as EPS (Encapsulated Postscript Vector graphics).
I didn't come up with this solution myself, but am grateful for finding it and would like to disseminate it wider - I know there are more of you struggling with the same problem! Dr. Hocking (Frostburg State University) published the solution on his website: click here.