You can save a plot as a PNG image in MATLAB using the saveas function. Here's an example of how to do it: 

% First, create a plot
x = linspace(0, 2*pi, 100);
y = sin(x);
plot(x, y);

% Use the saveas function to save the plot as a PNG
saveas(gcf, 'myplot.png', 'png');

In this example, gcf is used to get the handle of the current figure, and 'myplot.png' is the name of the file you want to save the plot as. The third argument 'png' specifies the file format you want to save the image in. You can change 'png' to 'jpg' or 'bmp' to save the image in a different format.

Note that the saveas function saves the entire figure, including any axes labels, legends, and other annotations. If you only want to save the plot itself without any surrounding elements, you can use the print function instead: 

% First, create a plot
x = linspace(0, 2*pi, 100);
y = sin(x);
plot(x, y);

% Use the print function to save the plot as a PNG
print('myplot.png', '-dpng', '-r300');

In this example, 'myplot.png' is again the name of the file you want to save the plot as, and '-dpng' specifies the file format. The '-r300' option specifies the resolution of the image in dots per inch (dpi). You can change this value to adjust the resolution of the saved image.