library(raster)
library(spatstat)

# First set the geographic extent we'll be using
e <- extent(-20, 60, -40, 35)

# Then read in the Maximum Green Vegetation Fraction tiff and crop it
mgvf <- raster("MGVF_2001_30_arc_sec.tif")
mgvf.2001.africa <- crop(mgvf, e)

# Now let's create a window for in spatstat
SP.win <- as(e, "SpatialPolygons")
W <- as(SP.win, "owin")

# Finally, we create the .im object
mgvf.img <- as.im(X = "mgvf.2001.africa", W = W)

# Notice, there are no errors thrown. However, compare the plots below and see the loss of information:
plot(mgvf.2001.africa)
plot(mgvf.img)

library(geostatsp)
mgvf.img <- as.im(mgvf.2001.africa)

rotate = function(mat) t(mat[nrow(mat):1,,drop=FALSE]) # mat - matrix